Carbon fiber electrode

ABSTRACT

The invention provides an electrode for use in an electrochemical reactor. The electrode comprising a plurality of carbon fibers in close proximity to one another, each of the fibers being in electrical contact with at least several of the other carbon fibers for transmitting an electrical potential substantially throughout the electrode when the electrode is in use in the reactor.

This invention relates to improvements in electrochemical reactors, andmore particularly to an improved electrode for use in such reactors.

Electrochemical reactors are used in many different processes andconsequently there are a great number of different structures in use.Among these processes are electrochemical synthesis, electrolysis,electrorefining, electrowinning, electrometallurgy, electrogeneration ofchemical species, and electrochemical treatment of waste.

The efficiency of any electrochemical reactor is related closely to thecharacteristics of the working electrode and more particularly to thesurface area of this electrode and the distribution of electricalpotential on the surface of the electrode. If the surface area incontact with the electrolyte is maximised the electrode will be capableof creating a large mass transfer flux, and if the potential is constantacross the surface area, the electrode will be controllable to be highlyselective and thereby ensure that desired results can be achieved.Consequently because the economic and commercial viability of anyelectrochemical process depends to a large extent on the efficiency ofthe reactor, any improvement in the performance of a reactor wouldsignificantly influence the cost of operating the process.

Different electrode designs have been attempted to optimize reactorefficiency, but in general an improvement in surface area has notresulted in an improvement in potential distribution and vice versa. Onedesign provides an electrode consisting of a particulate bed. Asignificant increase in surface area is achieved but the potentialdistribution is somewhat unpredictable due to poor mechanical contactwhich results in a potential drop between adjacent particles. As aresult the potential distribution varies and consequently the advantagesresulting from increased surface area tend to be diminished or cancelledby the poor potential distribution. An example of such a structure isfound in U.S. Pat. No. 3,827,964 to Katsuhiro Okubo et al.

Another design provides a fluidized bed electrode. Here again poorelectrical contacts between the particles results in inter-particulatepotential drop with consequent extreme non-uniformity of potential overthe fluidized bed electrode. Nevertheless the potential is notsufficiently predictable to permit accurate control of the electrode sothat the advantages of improved surface area are again offset by poorpotential distribution.

Apart from attempts to use various forms of granular or particulatebeds, the approach to optimizing electrode efficiency has been to devisevarious arrangements of sheets and the like to create multi-plateelectrodes separated by small spaces. Apart from the poor area achievedwhen compared with granular beds, the structures suffer from a furtherdisadvantage in that the small spaces tend to become blocked and aredifficult to arrange for adequate electrolyte flow. Most designs of thistype which offer reasonable efficiency include devices for stirring theelectrolyte or otherwise forcing the electrolyte through the electrode.

It is an object of the present invention to provide an improvedelectrode for use in an electrochemical reactor, the electrode havingenhanced potential distribution characteristic as well as a high surfacearea to volume ratio.

Accordingly the invention provides an electrode for use in anelectrochemical reactor, the electrode comprising a plurality of carbonfibers in close proximity to one another, each of the fibers being inelectrical contact with at least several of the other carbon fibers fortransmitting an electrical potential substantially throughout theelectrode when the electrode is in use in the reactor.

In this context the term "carbon fiber" is used to describe all fiberswhich are prepared by various heat treatment methods from suitableorganic precursors such as rayon or polyacrylonitrile fibers.

The invention will be better understood with reference to the drawingsin which:

FIG. 1 is a diagrammatic representation of a cross-section of anelectrochemical reactor incorporating an embodiment of an electrodeincorporating the inventive concept;

FIG. 2 is a diagrammatic representation of a cross-section throughanother embodiment of a reactor incorporating an electrode within thescope of the invention;

FIG. 3 is a further diagrammatic view illustrating another embodiment ofan electrode;

FIG. 4 is a perspective view of another electrode according to theinvention;

FIG. 4a is a side view of yet another electrode according to theinvention;

FIG. 5 is a somewhat diagrammatic sectional view of a reactorincorporating the electrode shown in FIG. 4;

FIG. 6 is an elevation of yet another embodiment of an electrode; and

FIG. 7 is a sectional view of a reactor using the electrode shown inFIG. 6.

Reference is first made to FIG. 1 which illustrates a simple embodimentof a reactor 20 including an electrolyte container 22 which is open atits top and which defines an electrolyte outlet 24 adjacent the opentop. This outlet controls the electrolyte level in the container 22.

The reactor also includes a working electrode 26 filled with carbonfibers 27 and located centrally within the container 22. The electrode26 is positioned inside a tubular counter electrode 28 and has a housing29 which includes a generally cylindrical main portion 30 of asbestos orthe like having a bottom 32, and a top assembly 34. The portion 30defines a plurality of small openings 35 adjacent bottom 32 to permitelectrolyte to escape from the housing 29. The openings are smallcompared with the length of a carbon fiber 27 so that the fibers tend tolie across the openings so that there is little likelihood of a fiberescaping from the housing 29.

The top assembly 34 consists of an annular element 36 which fits snuglyin the upper end of the main portion 30 and which is attached thereto byany suitable adhesive.

The element 36 in top assembly 34 is threaded internally to receive anexternally threaded compression element 38 having an upper squareportion 40 for combining with a wrench to turn the element 38 in theannular element 36. A lock nut 42 is also provided and this serves toretain an electrical connector 44 in position as well as to lock thecompression element 38 in a preferred position in the annular element36. The connector 44 is coupled to an end of an insulated conductor 46.

The counter electrode 28 includes an upwardly extending portion 48 whichis connected by a fastener 50 to a further electrical connector 52. Thisconnector receives another insulated conductor 54 which, as indicateddiagrammatically, forms part of an electrical power circuit having apower source 55 and which is also connected to the conductor 46.

An electrolyte inlet 56 is provided to carry electrolyte directly fromoutside the reactor 20 into the inside of the working electrode 26. Herethe electrolyte meets the carbon fibers before passing through openings35 in the main portion 30 and then moving past the counter electrode 28and hence out through the outlet 24.

The electrode 26 is shown in a form in which the assembly is carried outas follows. Firstly the carbon fibers are positioned in the main portion30 and then the top assembly 34 is engaged on the upper extremity of themain portion 30. For convenience, the carbon fibers are bunched quiteclosely and have a length comparable with that of the main portion 30.Once the annular element 36 is in place on the portion 30, thecompression element 38 is threaded into the element 36 using a wrench onthe square portion 40 of the element 38. This creates a compression inthe carbon fibers sufficient to hold them in compressive engagement withone another because although the fibers are of small diameter, they arequite rigid for their size. Consequently, with adequate packing in thehousing 29 the fibers will engage one another when a compressive load iscreated at one end of the housing thereby ensuring electrical contactbetween the fibers. Also, because of the negligible electricalresistance of the fibers and the numerous contacts between any one fiberand adjacent fibers, there is effectively a constant electricalpotential throughout the fibers. Once the compression has been achieved,the electrical connector 44 is engaged in the compression element 38 andthe lock nut 42 is engaged to hold both the element 38 and the connector44 in position.

In use, a power supply will be chosen according to the process beingused. If cations are to be collected, then the working electrode 26 willbe the cathode. Conversely, if anions are to be collected, then theworking electrode will be the anode.

The form of the working electrode can be varied without deviating fromthe present invention. The length of the fibers is preferably comparableto that of the length of the main portion 30. However the length can bevaried consistent with providing sufficient electrical contacts betweenthe fibers without severely restricting the flow of electrolyte betweenthe fibers. It will be evident that if the length of the fibers isreduced, a size will be approached where the compaction of the fiberswill severely affect the flow of electrolyte.

It has been found that carbon fiber electrodes of the type described areextremely efficient due primarily to the negligible electricalresistance within each fiber so that a substantially constant potentialcan be achieved in the fibers, and also because of the extremely largesurface area achieved by the use of these fibers in a given size ofelectrode. Typical suitable carbon fibers are made by CourtauldsLimited, Carbon Fibers Unit, Coventry, England, and sold under the trademark GRAFIL. Each of these fibers is typically about 5 to 15 microns indiameter although other sizes are available. The fibers are sold in atow each of which contains 5,000 or 10,000 fibers. Consequently it isconvenient to develop an electrode of the type described by taking partof the tow and placing it in a container such as housing 29 and thenapplying an end force to bring the fibers into intimate electricalcontact with one another. However for a given size of housing the masstransfer will be diminished as the cross-section of the fibers increasesbecause the surface area is effectively reduced.

It has also been found that carbon fibers have properties which areunexpectedly favorable to the efficiency of electrochemical processesgenerally. The fibers have low adsorption characteristics and tend to befree of harmful film formation at the surface of the fiber within arelatively large operating potential range. Consequently relatively highmass transfer rates can be achieved due to these properties combineswith the large surface area provided by the fibers per unit of volume ofthe electrode.

A further property of significance is the favorable overvoltagecharacteristics. The hydrogen and oxygen overpotentials are large(particularly for neutral solutions) so that carbon fibers can be usedboth as an anode and a cathode over a considerable potential range. Thismakes it possible for a large number of electrochemical reactions totake place on a carbon fiber electrode.

These examples typify results obtained using carbon fiber electrodes.

Although an electrochemical diaphragm has not been used in the exemplaryreactor, it will be evident that such a diaphragm can be used where aparticular process demands such use. Similarly, if a reactor is to beused without continuous flow, then the electrolyte flow could bediscontinued by dispensing with the inlet 56 and the outlet 24.

In some electrochemical processes the reactor described with referenceto FIG. 1 may not be suitable. Some examples of other forms which couldbe used are shown in FIGS. 2 to 4.

As seen in FIG. 2, a working electrode 58 is in the form of a mat madeup of many carbon fibers and positioned in an inclined tank 60. Some ofthe fibers extend outside the tank 60 to provide a terminal 62 forconnecting to a power source 63. The power source 63 is also connectedto a plate counter electrode 64.

Electrolyte (which may be waste effluent or the like) runs into the tank60 from an input stream 66 and mingles with the working electrode 58before leaving in an output stream 68. The residence time is controlledby the volume rate of flow permitted in the flow of the input stream 66.However the rate of flow must not be too large or the carbon fiberelectrode could be broken up and forced out of the tank 60.

As seen in FIG. 3, another embodiment of a reactor 70 consists ofrespective upper and lower halves 72, 74 which combine to define acavity 76 in which a plurality of carbon fibers 78 are contained. Thesefibers lie generally in side-by-side arrangement and some of them extendoutside the cavity 76 to provide a connection 80 for a power source 81as indicated. The upper half 72 defines a manifold opening 82 forfeeding electrolyte into many small openings 84 (some of which areshown) and which communicate with cavity 76. The openings 84 are smallto ensure a more even flow over the electrode and to prevent all of theflow taking place at one end of the electrode.

After passing over the fibers 78, the electrolyte leaves by way of smallopenings 86 leading to a manifold opening 88 in lower half 74. Theopening 88 contains a counter electrode 90 which is also connectedelectrically to the power circuit.

As previously mentioned it is well recognized that the efficiency andthe accuracy of control of the electrode depend on the potential drop inthe electrode and also on the surface area in contact with theelectrolyte. Also, however the electrolyte should move freely throughthe electrode while contacting as much of the surface area of theelectrode as possible. A particular form of carbon fiber electrode whichis advantageous in permitting such free movement of electrolyte is shownin FIG. 4.

Examples of the use of a working electrode of the type shown generallyin FIG. 3 will now be described. All electrode potentials mentioned aremeasured against a saturated calomel electrode.

EXAMPLE 1

A dark blue solution of copper sulphate having a concentration of 10,000part per million (p.p.m) was introduced to the reactor. The potential ofthe working electrode was held at -1.2 volts and the effluent solutionwas colourless indicating that the copper had been retained by theworking electrode. The working electrode potential was then switched to+0.2 volts and the effluent collection was blue indicating that copperwas being stripped from the working electrode.

The experiment was again repeated by feeding a solution of 10,000 p.p.m.copper sulphate to the reactor and again the effluent was colourless.Water containing some electrolytes was then introduced and the outputwwas again colourless. With water continuing to be introduced theworking electrode potential was switched to +0.2 volts and the outputsolution was blue indicating that copper was being stripped from theworking electrode.

Each of the conversions took place in a residence time of about 12seconds. The fact that the effluent in both cases was colorlessindicates a reduction from 10,000 p.p.m. copper to less than 400 p.p.m.The residence time will vary depending to some extent on the packing ofthe carbon fibers.

EXAMPLE 2

A solution containing 250 p.p.m. of lead was prepared and introduced tothe reactor. The effluent was collected and analysed by atomicabsorption spectrophotometry. This analysis was performed utilizing aPerkin Elmer atomic absorption spectrophotometer equipped with a MossmanFurnace atomizer. The following results were obtained.

The working electrode was held at -1.2 volts. The output had aconcentration of 0.2 p.p.m. lead.

The total conversion in about 12 seconds residence time was therefore99.9 percent in a single pass.

EXAMPLE 3

Example 2 was repeated for copper. The working electrode had a potentialof -1.2 volts and the input solution was copper sulphate. The inputsolution had a concentration of 250 p.p.m. copper and the output had aconcentration of 4 p.p.m. copper. Thus there was a 98.4 percentconversion in about 12 seconds residence time in a single pass throughthe reactor.

EXAMPLE 4

Example 2 was repeated for nickel. Nickel chloride was used with aworking potential of -1.6 volts. The input solution had a concentrationof 250 p.p.m. nickel and the output had a concentration of 20 p.p.m.Thus a 92 percent conversion was achieved in about 12 seconds residencetime in a single pass.

Preliminary studies have suggested that the electrochemical reduction ofnickel at carbon fiber electrodes is dissimilar to that of the othermetal reductions used in previous examples. The kinetics of the reactionare slower. This would explain the lower conversion factor of 92 percentin 12 seconds. A longer residence time may well achieve higherconversions in a single pass although the conversion factor is highlysatisfactory considering the short residence time in the reactor.

EXAMPLE 5

Solutions containing 500 p.p.m. and 100 p.p.m. of cadmium wereintroduced to the reactor and the effluent was monitored by on-lineautomated anodic stripping voltametry. Conversions of 99 percent wereachieved when the working electrode was at -1.4 volts.

As can be seen, the results from the examples show that efficiencies inthe order of 99 percent with a 12 seconds residence time can beachieved. This is both because the negligible resistance of the carbonfibers allows the potential across the electrode to be controlledprecisely, and also because of the high surface area to volume ratio ofthe electrode. Consequently, the two prime requisities of an electrodeused in removing metals from solution have been used in this structure,namely a very large surface area combined with a uniform electricalpotential across the area.

As seen in FIG. 4, an electrode 92 consists of a tow 94 engaged in acoupling 96 at a discrete upper end portion of the tow 94. This couplingalso includes an upstanding electrical terminal 98 for connecting theelectrode to an electrical power supply.

The tow 94 consists of numerous carbon fibers 100 all of which arepreferably substantially the same length and which lie generally inside-by-side arrangement. Respective corresponding ends of the fibers100 are co-terminus at the upper extremity of a ring 102 forming part ofthe coupling 96. This ring 102 has been deformed inwardly into firmengagement with the tow so that the individual fibers within the ringare in electrical contact with one another. Consequently, because thering is conductive, a potential applied at the terminal 98 will betransmitted by way of the ring 102 to the individual carbon fibers 100so that the potential on each of the fibers will be substantially thesame as that on all of the other fibers. Further, because the potentialdrop in the fibers is negligible, the potential at any point in thefibers will be substantially the same as the potential applied to thetow.

It has been found that carbon fiber electrodes of the type shown in FIG.4 are extremely efficient due to the aforementioned properties of carbonfibers and also because of the extremely large surface area incontinuous contact with electrolyte. Further in moving electrolyte thefibers tend to lie in the path of the flow to thereby ensure maximumcontact time as the electrolyte flows along the fibers.

The coupling 96 can take any suitable form consistent with maintainingthe fibers in their relative positions over a discrete portion of thelength of the tow while also permitting a potential to be applied tofibers consistently. In fact in simple applications a binding around thetow would suffice with fibers forming part of the tow above the bindingbeing used for the electrical connection. Consequently the coupling 96can be generalised to be any arrangement which locates the fibersrelative to one another. Although the coupling 96 includes electricalconnection 98, many other arrangements can be used such as the simplebinding already mentioned.

As seen in FIG. 4a, the coupling 96 (FIG. 4) could be replaced in asimple form by a binding 96a which may or not be electricallyconductive, and by extending at least some of the carbon fibers past thebinding for use in making an electrical connection 98a.

A typical use of the electrode 92 shown in FIG. 4 is illustrated in FIG.5 in which the electrode is being used for electrowinning. Electrolyte104 is being fed through an inlet 106 so that the electrolyte initiallyenters an electrolyte guide 108 positioned about an electrode 110 ofsimilar form to that described with reference to FIG. 4. The guide 108is open at its bottom so that electrolyte can pass downwardly throughthe carbon fibers and then upwardly past a counter electrode 112 whichis also positioned inside an electrolyte container 114. An electrolyteoutlet 116 is provided adjacent the top of the container 114 and a powersupply 117 is connected to the working electrode 110 and counterelectrode 112.

In use, the individual fibers are free to flex in the stream ofelectrolyte as this electrolyte moves downwardly through the guide 108.Consequently, there is a tendency for the fibers to lie individually inthe electrolyte due to flow effects around the fibers. The tow will thentake up a position somewhat as that indicated in ghost outline in FIG.5. Because of this movement in the tow, it is preferable that the guide108 be non-conductive because it must be sufficiently close to the towto ensure that the electrolyte flow effects the tow. Further, as metalis deposited on the fibers due to the electrochemical process, the flowwill tend to maintain the separation between fibers within limits offlow rate and weight and fibers.

Although the process shown in FIG. 5 demonstrates the use of the fibersin electrowinning, it will be evident that a reactor such as that shownin FIG. 5 can be used in effluent treatment and control, andelectro-organic synthesis with or without conventional modifications tothe reactor such as the use of electrochemical diaphragms, and a thirdor reference electrode. For instance the guide 108 could be anelectrochemical diaphragm in which case the bottom of the guide would beclosed and an outlet 119 used as indicated in ghost outline. Similarlyan electrochemically compatible liquid would be fed into an inlet 121(also shown in ghost outline) and this liquid would leave by way ofoutlet 116.

Reference is now made to FIG. 6 which shows yet another form ofelectrode 118. This electrode consists of a tow 120 made up of numerouscarbon fibers 122 which are restrained in five discrete positions alongthe length of the fibers by couplings 124. The same potential isprovided at each of the couplings so that the longest electrical path isfrom a particular coupling to the mid point of the carbon fiber betweenthis coupling and an adjacent coupling.

The electrode shown in FIG. 6 is conveniently used in a reactor 126shown in FIG. 7. In this reactor electrolyte 128 is flowing into thereactor and is guided by baffles 130 towards the working electrode 132of the form shown in FIG. 6. This electrode lies transversely of thedirection of travel and the tow is slightly loose between the couplings124 to provide minor transverse oscillation of the fibers as theelectrolyte flows across the fibers. Subsequently the electrolyte movesdownwardly leaving the electrode 132 and passes a counter electrode 134under a major vertical baffle 136 which ensures that the electrolytefirst moves downwardly towards the counter electrode and upwardly backinto a stream 138. As illustrated, a suitable power supply 139 isprovided.

The main use of electrochemical reactors is in continuous processeswhere the electrolyte is changed continuously as was described withreference to FIGS. 5 and 7. However, in a reactor in which the electrodeis not changed, there would be no need to use a guide such as theelectrolyte guide 108 of FIG. 5. In such an embodiment, the electrode110 would have to be spaced from the counter electrode 112 sufficientlyto prevent a short circuit. Otherwise, the structure would be similar tothat shown in FIG. 5 with the exclusion of the electrolyte guide 108.Similarly, in a process such as that shown in FIG. 7 it would bepossible to use a bath in which the electrodes were contained, and afterfilling the bath, the flow would be curtailed and the electrochemicalprocess allowed to take place. Subsequently, the electrolyte would thenbe removed from the bath and a further charge of electrolyte enteredinto the bath. In such a system the baffles 130 and main baffle 136could be eliminated, although of course the arrangement of electrodescould also be changed because the reason for their location in FIG. 4 isno longer pertinent.

Although several forms of electrode have been described, numerous otherforms are possibly consistent with the use of carbon fibers. Such otherforms are within the scope of the inventive concept as described andclaimed.

Throughout the description, the counter electrode and the electrolytecontainer have been shown as separate elements. It is intended that suchan arrangement when described and claimed will include the equivalentstructure (where applicable) of a container which either doubles as anelectrode or which includes such an electrode in the structure of thecontainer.

What we claim is:
 1. A flow-through electrode for use in anelectrochemical reactor, the electrode comprising:means providing forelectrical connection to the electrode; a housing having a preferredlength; a plurality of individual carbon fibers arranged contiguously inthe housing and each of the carbon fibers having a length at least equalto said preferred length and having an end coupled to the electricalconnection means for electrical continuity between said means and thecarbon fibers; and said carbon fibers being sufficiently numerous toeffectively fill the housing, the housing having an inlet and an outletto direct a forced flow of electrolyte between the carbon fibers in thehousing for better electrochemical efficiency.
 2. A flow-throughelectrode as claimed in claim 1 in which the housing further includesmeans creating a compressive load on the carbon fibers whereby thecarbon fibers are more positively packed in the housing.
 3. Aflow-through electrode as claimed in claim 1 in which the housing is anelectrochemical diaphragm.